(1) Field of the Invention
The present invention relates to a method and apparatus for photometering an image recorded on a film.
(2) Description of the Related Art
Generally, an exposure with which a color image is printed from a color original onto a photosensitive material is determined with respect to each of three primary colors R(red), G(Green) and B(blue) by measuring integral transmission (or reflection) density for respective colors R(red), G(Green) and B(blue) by means of a photometering apparatus equipped with color separation filter composed of colored dye filters and/or deposition filters. In order to determine an exact exposure, it .is necessary to measure a light amount actually affecting the photosensitivity of a photosensitive material. This requires that a spectral sensitivity distribution of the photometering apparatus be made coincident with that of the photosensitive material. The spectral sensitivity distribution of the photosensitive material is asymmetrical with respect to a wavelength at which the photosensitivity becomes maximum. To produce an asymmetrical distribution of transmittance using colored dye filters and/or deposition filters, however, requires complicated combinations of many filters. For this reason, it is difficult not only to mass-produce photometering apparatuses, but also to manufacture these with good precision.
To overcome this problem, in the art of a photoresist exposure apparatus, a technology is known in which light from an original is separated into spectral light waves and the thus separated light waves are weighted and added, so that the spectral sensitivity distribution of a photometering apparatus is made coincident with the spectral sensitivity distribution of a photosensitive material. As an example of this art, Japanese Patent Application Laid-Open No. 1-142719 discloses a method of color printing using a prism (or diffraction grating), lenses and two-dimensional array sensors and an apparatus using this method. In this method of color printing, an original is divided into a plurality of sub-sections, and light having passed through the sub-sections of the original is paralleled by lenses. Then the thus paralleled light is made incident upon a prism to be separated into a spectrum. The thus formed spectral light waves refracted at different angles in accordance with their respective wavelengths are emitted from the prism to be measured for light intensity by means of a two-dimensional array sensor. The measurement is conducted for each of the sub-sections of the original.
This method of color printing, however, requires the use of an expensive prism (or diffraction grating) resulting in high cost. Further, when a prism is employed, it is necessary to prepare a point light source through a slit in order to form the incident light on the prism into parallel light, resulting in loss of light utilization. Moreover, the above method makes use of refraction or diffraction of light, which disadvantageously makes the photometering structure bulky.